Accelerometers are sensors or transducers that measure acceleration. Accelerometers generally measure acceleration forces applied to a body by being mounted onto a surface of the accelerated body. Typical accelerometer sensors utilize a flexure assembly. More specifically, they may include a pendulous reaction mass (often referred to as a proof mass) suspended from a stationary frame by, for example, one or more flexural suspension members or some other form of pivot mechanism. The flexures enable the proof mass to deflect in response to forces or accelerations along a sensitive axis of the accelerometer, which is generally perpendicular to the plane of the proof mass. In general, the relative displacement of the proof mass is directly proportional to the acceleration of the accelerated body.
Various types of pendulous reaction mass accelerometers exist, including, for example, vibrating beam accelerometers, capacitive accelerometers, capacitive rebalance accelerometers, and translational mass accelerometers. A capacitive accelerometer, for example, features a capacitor between the proof mass and the stationary support structure (i.e., a first capacitor plate is coupled to the moving proof mass, while a second capacitor plate is coupled to the stationary support structure). An acceleration of the proof mass causes a change in the space between the moving and fixed plates of the capacitor, which changes the electrical capacitance of the capacitor and varies the output of an energized circuit. The change in the electrical capacitance of the capacitor is representative of the acceleration or force along the sensitive axis of the accelerometer.
Alternatively, in contrast to this open-loop operation, a force rebalance accelerometer keeps the proof mass in a state of equilibrium by generating a force (e.g., with a mechanical, electrical, or magnetic force generator) that opposes the specific force applied along the sensitive axis of the proof mass by the acceleration acting thereon. The amount of force that is generated by the force generator in order to keep the proof mass in its equilibrium state is indicative of the acceleration along the sensitive axis of the accelerometer.
Generally, it is desirable to have extremely low parasitic forces along the sensitive axis of the accelerometer because the parasitic forces cannot easily be differentiated from sensed accelerations. In addition, the flexures suspending the proof mass of the accelerometer are generally designed to limit motion to the unique sensitive axis of the accelerometer. Thus, high rigidity in the flexures in the directions orthogonal to the sensitive axis of the accelerometer is typically necessary in order to precisely define the sensitive axis.
FIG. 1 schematically depicts a portion of an accelerometer 100 that features ordinary, unstressed flexures 104 suspending a proof mass 108. The ordinary, unstressed flexures 104 can be made to be very flexible along the sensitive axis 112 of the accelerometer 100, but will always have a non-zero spring rate, as illustrated in FIG. 2. This non-zero spring rate, which may be viewed as a parasitic force acting along the sensitive axis 112 of the accelerometer 100, introduces an error term into the acceleration reading. The spring rate may be reduced by making the flexures 104 longer and/or thinner, but this will disadvantageously also reduce rigidity in the directions orthogonal to the sensitive axis 112.
Alternatively, high performance proof-mass based accelerometers may achieve a zero spring rate by using electric or magnetic fields, as opposed to unstressed flexures, to suspend the proof mass. However, electrically or magnetically suspended accelerometers are much more complicated and expensive than flexure suspended accelerometers.
Accordingly, a need exists for improved flexure suspended accelerometers and for methods of manufacturing and using the same.